1. Field of the Invention
The invention relates to a method for detecting identification media within the communication range of an antenna of a read/write unit according to the preamble of claim 1.
It is known that between an antenna of a read/write unit based on the principle of inductive coupling of an RF field and a conductive object, for example an identification medium, inductive coupling occurs in the vicinity of the antenna as soon as the RF field is switched on.
In RFID (radio frequency identification) systems, contactless communication between an identification medium and a read/write unit is based on this inductive coupling. In a read/write unit with power line connection in which the current consumption does not matter, the RF field can be switched on as often and as long as desired in order to look for an identification medium within the communication range and to set up a communication. Accordingly, communication is set up in such read/write units by means of a communication signal for authenticating a passive identification medium. For this purpose, the RF field is switched on, e.g. every 200 ms, an authentication signal (with modulation) of some ms duration is sent out and a response is awaited. Because of the relatively high current consumption, this method is not optimal for battery-operated read/write units. In this case, it would be very advantageous to send the communication signal only when an identification medium is located within the communication range of the antenna of a read/write unit. The problem is to detect when this is the case. Proximity detectors for solving this problem, e.g. optical ones, require additional circuits and respond to any objects—they cannot specifically indicate an identification medium or its coupling to the RF field.
2. Description of the Art
From EP 0 944 014, a method is known which is intended to enable identification media to be detected within the vicinity of a read/write station, but only in the lower RF band, i.e. in the previous standard band of 125 KHz. However, this method is not applicable to high-power RFID systems with carrier frequencies in the MHz band, preferably above 5 MHz or 10 MHz and especially at 13.56 MHz where much higher information transmission rates and much more sophisticated and further applications are possible than in the standard 125 KHz band. Such high-power systems in the MHz band are known, e.g. from WO 97/34265.
The method according to EP 0 944 014 is based on the excitation of the resonance frequency of an antenna by means of a single pulse and measuring the decay characteristic of this signal. In this method, a short rectangular single pulse of, e.g. 2 μs duration (i.e. much shorter than a fundamental oscillation of approx. 8 μs at 125 KHz) is generated with reduced current and used for exciting a transmitting antenna into natural oscillation at its resonance frequency. After a waiting time of, e.g. 200 μs (corresponding to approx. 25 fundamental oscillations), during which the single signal decays, the decayed single-pulse signal is measured over, e.g. 20 μs via a receiving antenna. With an identification medium in the vicinity of the read/write unit, the signal decays more strongly than without. Correspondingly, the presence of an identification medium is concluded if the single-pulse signal or the natural oscillation decays below a certain value.
However, this method for the 125 KHz band, could not be implemented at all for several reasons in high-power systems in the MHz band: a single pulse which is much shorter than the fundamental oscillation of, e.g. 0.1 μs at 10 MHz cannot be achieved and the decaying of a single pulse or of a natural oscillation which must occur 100-times more rapidly here than in the 125 KHz band, could not be measured at all, and an influence of an identification medium on the decay of a single pulse even less.
This method according to EP 0 944 014 has other disadvantages: generating a short single pulse which does not correspond to the fundamental oscillation for the RF communication requires an additional circuit. During the waiting time, this circuit must be active. It is not possible to detect identification media within the entire communication range in which the RF communication takes place with higher power, by means of a single pulse generated with reduced current. Neither is this known method suitable for detecting identification media with a resonant frequency which clearly differs from those of the antennas of the read/write unit.
In high-power systems in the MHz band, preferably at least 5 MHz or at least 10 MHz and especially at 13.56 MHz, the known microprocessors cannot be used for carrying out relatively slow measuring methods of the decay characteristic of natural oscillations for standard systems with 125 KHz by exciting the antenna at its natural frequency, switching off and measuring the decay characteristic of these natural oscillations within a measuring time of, e.g. 0.2-1 ms (e.g. by measuring a start value and an end value of the amplitudes within the measuring time or by counting the number of oscillations until they have decayed to a certain threshold value). In particular, e.g. in a 13.56 MHz system, the natural oscillation must have decayed completely to the value 0 within 2.4 μs so that communication can be carried out here. It would not be possible to measure any decay characteristic within this very short time.